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Old 04-05-2007, 10:02 AM   #1
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Deck footing below water table!


Hi all, I live in Northern Ontario Canada and I'm a first time home owner (ah thank you). I plan on building a nice big deck off the front of my house this spring. Although living on the shore of a lake is beautiful it does pose a problem with deck footings. I'm told that I need to dig down at least 6 feet to be below our frost line. However, once I'm 2 feet down I'm into the lakes water table and the hole fills with water. I'm stumped, what do I do with this situation, how do i get good strong and solid footings in water filled holes?

I've heard of people wrapping the base of their SonoTube with thick plastic then pouring cement in. Is that a sufficient solution or is there any other suggestions? ANY suggestions anybody may have please do share the with me. Thank you so much!!

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Old 04-05-2007, 11:07 AM   #2
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Deck footing below water table!


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Originally Posted by flyultralite View Post
Hi all, I live in Northern Ontario Canada and I'm a first time home owner (ah thank you). I plan on building a nice big deck off the front of my house this spring. Although living on the shore of a lake is beautiful it does pose a problem with deck footings. I'm told that I need to dig down at least 6 feet to be below our frost line. However, once I'm 2 feet down I'm into the lakes water table and the hole fills with water. I'm stumped, what do I do with this situation, how do i get good strong and solid footings in water filled holes?

I've heard of people wrapping the base of their SonoTube with thick plastic then pouring cement in. Is that a sufficient solution or is there any other suggestions? ANY suggestions anybody may have please do share the with me. Thank you so much!!
I have my own comments about how to deal with this...but I'd rather hear from an expert in the area ... like....CONCRETEMASONRY...
Hopefully, he'll see this and reply...

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Old 04-05-2007, 11:19 AM   #3
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Deck footing below water table!


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I have my own comments about how to deal with this...but I'd rather hear from an expert in the area ... like....CONCRETEMASONRY...
Hopefully, he'll see this and reply...

I don't think there's any harm in hearing your comments.
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Old 04-05-2007, 12:01 PM   #4
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Deck footing below water table!


The first thing is to makes sure you have solid soil where you plan to have your Sonotubes sit. If it is bad, it makes little difference how you pour the concrete.

I am not an expert on digging and every job seems to be different depending on the soil and water level. It is a waste of time trying to keep the water out of the excavation. In some cases you just have to dig and insert the tube ot sometimes you can jet the bottom soil loose with a hose and force the tube downward is steps.

To pour the concrete below water, do the same think they do on bridge seals but on a smaller scale. On bridges they may pour 10 to 20 feet or more below water. You just the same principle, but on a smaller scale.

With the hole dug, the Sonotube in place (vertical and set to height), and water in the Sonotube, insert a large hose or pvc pipe (3-4") down to the bottom of the Sonotube form. Pour concrete(3/4" maximum aggregate size) down the tube - it is heavier and will force the water up. The beginning of the pour will loosen a lot of poor soil and fines and they will be forced upward. You must keep the bottom of the pipe well below the top of the concrete. As you pour the concrete, the water will be displaced and forced out of the top of the form. When you get the form filled, scrape off the extremely wet concrete and pour more into the pipe to get the better mix to the top.

Insert some vertical rebar and a Simpson bracket that you can attach your wood post to.

Since you will get little lateral support from a concrete tube 6' into wet, mucky soil, you will have to rely on some diagonal bracing of the deck in both directions.
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Old 04-05-2007, 01:07 PM   #5
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Deck footing below water table!


It is very scary to pour anything below the water table. Where I live, there are some areas near the bay that are at sea level. It is a bad idea to build swimming pools there, but someone had a Gunnite (concrete w/ rebar) pool put it anyway. Guess what? After a few months, this very very heavy concrete pool with water heaved up and cracked. The entire $45,000 pool is useless now.

If you're using a ledger board instead of building a free standing deck, when the deck moves, it may affect the structural integrity of your house also. What you may have to do is to have a concrete pad poured on that area and then install the deck posts on top of the concrete pad. I assume your lake house is built the same way where there's no basement and only a concrete slab.
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Old 04-06-2007, 08:14 AM   #6
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Deck footing below water table!


What I'm doing actually is taking off the exisitng sunroom. It was built in '73 (very poorly) and needs major repair. I figured I'd get more use with a deck anyways.

The exisiting sunroom in essence is a deck, just with walls and a roof. It's attached to the house with a ledger and has 4 posts resting on footings. We still have snow so I can't dig down to see the depth of their footings. The sunroom has never heaved all these years, everything is nice and level still.

With the above knowledge would it be wise to duplicate exactly what they did back in '73? If thier footings are only 2 feet below ground and hasn't moved, should I do the same? Which is better, being way above frost line and OUT of the water table or below frost line and IN the water table?
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Old 04-06-2007, 09:19 AM   #7
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Deck footing below water table!


If the existing footers are above 2 ft, then I think whoever built that sunroom probably realized the same thing while digging to install the posts. That's why he kept it above the frost line. That's assuming the lake's water level hasn't changed.

One important thing to factor in is that the sheer weight of the sunroom may be greater than your deck, as it includes framing, windows, and roofing. So, what did not heave before, may still move since your deck will be lighter. Duplicating exactly what they did then may or may not work.

I don't know how high your deck or sunroom is, but I assume the grade slopes down towards the lake. Worse case, you can build a retaining wall to level out the area just below your deck. With all the fill dirt needed to level things out, your new footers will be techinically be a bit lower below the new frost line, and you'll have the benefit of a flat, usable area below your deck, especially if you use pavers to make a patio.

Last edited by handy man88; 04-06-2007 at 09:56 AM.
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Old 04-06-2007, 10:18 AM   #8
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Deck footing below water table!


Maybe look into pilings that dock builders use. These guys are near Ottawa , maybe they can advise you.
http://www.tmpo.ca/home.php?lang=en

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